1. Technical Field of the Invention
The present invention relates to an oxide single crystal and a method of manufacturing it. More particularly, the oxide single crystal has the crystal structure of Ca.sub.3 Ga.sub.2 Ge.sub.4 O.sub.14 and contains Ge as a constituent element and the method comprises the steps of preparing a melt of starting materials containing Ge, and growing said oxide crystal from said melt of starting materials in single-crystal growing atmosphere. The oxide single crystal is used as a material for piezoelectric devices (because of its piezoelectric properties) and a solid laser host material.
2. Related Art of the Invention
Since the discovery of Ca.sub.3 Ga.sub.2 Ge.sub.4 O.sub.14 compound by B. V. Mill et al., a number of compounds of the same structure have been found as the result of substitution of constituents with other cations. Examples are La.sub.3 Ga.sub.5 SiO.sub.14, La.sub.3 Ga.sub.5.5 Nb.sub.0.5 O.sub.14, La.sub.3 Ga.sub.5.5 Ta.sub.0.5 O.sub.14, and La.sub.3 Ga.sub.5 ZrO.sub.14 which are formed by substitution of Ca with La or substitution of Ge with Ga and Si, Ga and Nb, Ga and Ta, or Ga and Zr. They have been found to have electromechanical coupling coefficients greater than that of .alpha.-quartz. They have also been found to have a crystal axis along which the temperature coefficient of frequency is zero. For these reasons, they are regarded as a promising material for the piezoelectric devices of the next generation of mobile communications equipment. The technology in this field has advanced to such an extent that in a material without Ge, La.sub.3 Ga.sub.5 SiO.sub.14, a single crystal (an ingot having about 3 inches in diameter) can be produced easily by Czochralski method.
It is known that the La,Ga-based compound mentioned above is surpassed in electromechanical coupling coefficient by those Ca.sub.3 Ga.sub.2 Ge.sub.4 O.sub.14 compounds in which the Ca site is replaced by an alkali metal ion or alkaline earth metal ion. Nevertheless, these compounds are not yet available in the form of a large single crystal because of many difficulties involved. For example, they have a melting point of 1300.degree. C. or above which exceeds the vaporizing point of GeO.sub.2, one of the starting materials. This leads to a product of poor quality due to its composition differs from that of the starting materials, cracking in the crystal, and gaseous impurity phases.